This invention relates generally to fiber optic communication. More particularly, it relates to safety mechanisms for Raman pump sources.
Modern communications systems often rely on optical fibers to transmit signals over long distances. Multiple fibers can be bundled together to form a fiber cable. An optical fiber can carry a greater density of signal channels than an electrically conductive wire of the same thickness. Optical communications systems sometimes reserve one of the signal channels for the purpose of communicating system parameters such as component temperature, pump power, and on/off status. Because the optical signal tends to attenuate over the length of the fiber, optical communications systems often employ amplifiers every few kilometers along a given fiber to maintain the signal intensity above some noise level. The amplifier boosts the level of the signal for transmission along the next segment of fiber. A recent development in the field of optical communications is the use of a Raman pump to amplify optical signals. A Raman pump provides radiation having a wavelength in vacuum typically between 1200 nanometers and 1500 nanometers. The pump radiation pumps energy into an optical signal in the fiber through a core field that is distributed along the length of the fiber. The Raman pump radiation usually propagates in a direction counter to the direction of propagation of the optical signal. As a result the intensity of an optical signal tends to decay at first and then increase in intensity as it approaches the Raman pump.
Unfortunately, the level power produced by a Raman pump presents a potential safety hazard. An optical power above 5 mW is considered eye-unsafe. A Raman pump, however, produces radiation at a very high power level, typically 200 mW to 2 Watts. This level of power represents an extreme eye-safety hazard should a fiber carrying Raman pump radiation be accidentally cut or disconnected. Thus a Raman-pump-based optical communications system preferably includes a fail-safe mechanism that turns off the Raman pump in the event of an accidental cut or disconnection of the fiber. However, prior art eye safety systems have not yet considered the extreme hazard present by the Raman pump.
For example, U.S. Pat. No. 5,945,668 describes a fiber optic safety interlock that uses a cable having a wire and an optical fiber. The wire carries a signal from a transmitter to a receiver. The receiver is coupled to a power supply that provides optical power to the fiber. If the fiber is cut the wire is also cut and the receiver turns off the power supply. Unfortunately this system is neither fail-safe nor reliable. There may be a considerable delay between the time the fiber is cut and the time the wire is cut. The delay may be long enough to cause eye damage. It is also possible that the wire may not be cut when the fiber is cut. Furthermore, it is possible that the wire may be cut when the fiber is not cut. Such a situation would trigger a false alarm.
U.S. Pat. No. 5,428,471 describes a fail-safe automatic shutdown system for a two-way fiber optic communications system. This system uses a pair of fibers with each fiber in the pair carrying signals in a direction opposite that of the other fiber. The system uses continuity signals transmitted over both fibers to shut down amplifiers on the upstream side of a cut in one of the fibers. The system relies on a cut in one fiber to trigger a shutdown of a first amplifier on the other fiber. The shutdown of the first amplifier triggers a shutdown of a second amplifier coupled to the fiber that was cut. Under certain circumstances a signal triggering a shutdown might travel the entire length of the fiber twice before the system actually shuts down. Furthermore, such a system will not work with a single fiber.
U.S. Pat. No. 5,136,410 describes an optical fiber link control safety system for two-way traffic on a single fiber. This system uses the optical signals carried by the fiber to trigger a shutdown of signal power if the fiber is cut. However, at certain times there may be no signal at all on the fiber, e.g., when all signal channels are dropped. This is not a problem for signal power. However, in Raman pumped systems, is the Raman pump is usually on whether a signal is present or not. If a fiber cut occurs when the signal power is off and the pump is on the Raman pump will not be shut off.
There is a need, therefore, for a reliable Raman safety latch that shuts off the pump in minimal time in the event of a cut or disconnection of the fiber.
Accordingly, it is a primary object of the present invention to provide a safety latch that turns off the Raman pump quickly enough to prevent eye damage in the event of a fiber cut. It is a further object of the invention to provide such a safety latch that operates in a fail-safe mode. It is an additional object of the invention to provide safety latch that detects a cut in a fiber without requiring the use of an additional fiber or wire. It is a further object of the invention to provide a safety latch for a Raman fiber pump that operates reliably while reducing the probability of a false alarm.
These objects and advantages are attained by a method and apparatus for shutting off optical power to an optical fiber in the event of a cut in the fiber. According to a first embodiment, the method generally comprises providing a supervisor signal to a first end of the fiber and shutting off the optical power to the fiber in response to a loss of or change in the supervisor signal at a second end of the fiber. The supervisor signal is generally operated in an xe2x80x9calways-onxe2x80x9d mode to provide for fail-safe operation. The supervisor signal may also be pulsed, i.e., quasi always-on. The method is particularly applicable to high power (e.g.,  greater than 200 milliwatts (mW)) source such as a fiber pump. It is also desirable that the supervisor signal be an eye-safe signal to reduce hazards to eye-safety. In a particular embodiment, the optical power source is a Raman pump. In the present method it is desirable to turn off the power to the fiber within an eye-safe time after the fiber has been cut. To this end it preferable that the supervisor signal propagate in a direction opposite that of radiation from the optical power source. The supervisor signal may include an auxiliary supervisor signal, such as an optical communications signal carried by the fiber or optical power from the optical power source, where the optical power has been reflected from a cut in the fiber.
A second embodiment of the invention provides an apparatus for shutting off a source of optical power to a fiber in the event of a cut in the fiber. The apparatus generally comprises a controller coupled to the source of optical power and means for coupling a supervisor signal, which has been provided to a first end of the fiber, to the controller. The source of optical power is operable in response to signals from the controller. The controller responds to the supervisor signal by shutting off the power source if the supervisor signal is not present at a second end of the fiber.
The method of the first embodiment and/or apparatus of the second embodiment may be incorporated into an optical communications system according to a third embodiment of the invention. The optical communications system generally comprises an optical fiber and two or more hubs coupled to the fiber. A first hub includes a Raman pump coupled to the first end of the fiber and a controller coupled to both the first end of the fiber and the Raman pump. A second hub includes an optical signal source coupled to the second end of the fiber and a supervisor signal source coupled to the second end of the fiber. The controller shuts off pump radiation from the Raman pump to the fiber if the Raman pump is on and a supervisor signal from the supervisor signal source is not present at a first end of the fiber.
The method of the first embodiment may be implemented in the form of a processor readable code embodied in a processor readable medium. The code may operate a processor controllable apparatus according to a fourth embodiment of the invention. Such an apparatus may be part of an optical communications system, such as the system of the third embodiment.
The various embodiments of the present invention provide for a fail-safe means of turning off a source of optical power and thereby reducing an eye-safety hazard resulting from a cut in an optical fiber. By using a supervisor signal separate from the optical communications signals carried by a fiber optic network, the system is less susceptible to false alarms than prior art systems. Furthermore, if the supervisor signal propagates opposite the radiation from the source, the various embodiments respond faster to a cut in the fiber.